Microspheres

ABSTRACT

The invention relates to thermally expandable thermoplastic microspheres comprising a polymer shell made from ethylenically unsaturated monomers encapsulating a propellant, said ethylenically unsaturated monomers comprising from 40 to 70 wt % of acrylonitrile, from 5 to 40 wt % of methacrylonitrile, from 10 to 50 wt % of monomers selected from the group consisting of esters of acrylic acid, esters of methacrylic acid and mixtures thereof, and said propellant comprising at least one of methane, ethane, propane, isobutane, n-butane and neo-pentane. The invention further relates to the production and use of the microspheres.

The present invention relates to thermally expandable thermoplasticmicrospheres, production and use thereof, and an aqueous slurrycomprising such microspheres.

Expandable thermoplastic microspheres comprising a thermoplastic polymershell encapsulating a propellant are commercially available under thetrademark EXPANCEL® and are used as a foaming agent in many differentapplications.

In such microspheres, the propellant is normally a liquid having aboiling temperature not higher than the softening temperature of thethermoplastic polymer shell. Upon heating, the propellant evaporates toincrease the internal pressure at the same time as the shell softens,resulting in significant expansion of the microspheres. The temperatureat which the expansion starts is called T_(start), while the temperatureat which maximum expansion is reached is called T_(max). Expandablemicrospheres are marketed in various forms, e.g. as dry free flowingparticles, as an aqueous slurry or as a partially dewatered wet-cake.

Expandable microspheres can be produced by polymerising ethylenicallyunsaturated monomers in the presence of a propellant. Detaileddescriptions of various expandable microspheres and their production canbe found in, for example, U.S. Pat. Nos. 3,615,972, 3,945,956,4,287,308, 5,536,756, 6,235,800, 6,235,394 and 6,509,384, in EP 486080,EP 1054034, EP 1288272 and EP1408097, in WO 2004/072160, and in JP LaidOpen No. 1987-286534.

One important application for expandable microspheres is paper making asdescribed in e.g. U.S. Pat. Nos. 3,556,934 and 4,133,688, JP Patent2689787, JP Laid Open No. 2003-105693, WO 2004/113613, WO 2006/068573and WO 2006/068574, and in Ö. Söderberg, “World Pulp & Paper Technology1995/96, The International Review for the Pulp & Paper Industry” p.143-145.

Other important applications for expandable microspheres are printinginks, vinyl foams (e.g. plastisols), non-woven and artificial leather.

In some applications it is desirable that the microspheres have acomparatively low T_(start). However, the polymer shell in commerciallyavailable microspheres with a low T_(start) are usually made of amonomer mixture comprising halogen containing monomers like vinylidenechloride. Such microspheres usually suffer from high amounts of residualmonomers, discolouration and poor resistance to chemicals, like solventsand plasticisers used in artificial leathers and plastisols. Attempts tomake microspheres with low T_(start) and high expansion capabilitywithout halogen containing monomers have not yet solved these problemssatisfactorily

It is an object of the invention to provide expandable microspheres withhigh expansion capability and low T_(start) without high amounts ofhalogen containing monomers.

It is another object of the invention to provide expandable microsphereswith low T_(start), high resistance to chemicals and high brightness.

It is still another object of the invention to provide expandablemicrospheres useful in paper making or in printing inks, for example asa foaming agent therein.

It is a further object of the invention to provide a process for theproduction of paper.

It is still a further object of the invention to provide an aqueousslurry comprising expandable microspheres useful in the production ofpaper.

It has surprisingly been found possible to fulfil these objects bycombining a certain monomer composition for the polymer shell with acertain group of propellants.

One aspect of the invention concerns thermally expandable thermoplasticmicrospheres comprising a polymer shell made from ethylenicallyunsaturated monomers encapsulating a propellant, said ethylenicallyunsaturated monomers comprising from 40 to 70 wt % of acrylonitrile,from 5 to 40 wt % of methacrylonitrile, from above 10 to 50 wt % ofmonomers selected from the group consisting of esters of acrylic acid,esters of methacrylic acid and mixtures thereof, and said propellantcomprising at least one of methane, ethane, propane, isobutane, n-butaneand neo-pentane.

The ethylenically unsaturated monomers preferably comprise from 45 to 65wt % of acrylonitrile. If a low T_(start) is of highest priority thecontent of acrylonitrile is most preferably from 45 to 55 wt %, and ifhigh resistance to chemicals is of highest priority the content ofacrylonitrile is most preferably from 55 to 65 wt %. The ethylenicallyunsaturated monomers further preferably comprise from 10 to 35 wt %,most preferably from 15 to 30 wt % of methacrylonitrile. Theethylenically unsaturated monomers further preferably comprise from 15to 50 wt %, most preferably from 20 to 40 of monomers selected from thegroup consisting of esters of acrylic acid, esters of methacrylic acidand mixtures thereof.

It has been found that if the ethylenically unsaturated monomerscomprise above 10 wt % of esters of acrylic or methacrylic acid, ormixtures thereof, it is possible to obtain microspheres with highexpansion capability and comparatively low T_(start).

Esters of acrylic and methacrylic acid preferably have only one carbonto carbon double bond. It has been found that esters of acrylic acidsuch as methyl acrylate, ethyl acrylate and butyl acrylate, inparticular methyl acrylate and ethyl acrylate, are especially favourableas monomers. The ethylenically unsaturated monomers thus preferablycomprise from above 10 to 50 wt %, most preferably from 15 to 50, wt %particularly most preferably from 20 to 40 wt % of monomers selectedfrom the group consisting of esters of acrylic acid. It is preferred ifthe ethylenically unsaturated monomers comprises from above 10 to 50 wt%, most preferably from 15 to 50, wt % particularly most preferably from20 to 40 wt % of monomers selected from the group consisting methylacrylate, ethyl acrylate and mixtures thereof. It is particularlypreferred if the ethylenically unsaturated monomers comprises from above10 to 50 wt %, most preferably from 15 to 50, wt % particularly mostpreferably from 20 to 40 wt % of methyl acrylate.

If included, esters of methacrylic acid may, for example, be one or moreof methyl methacrylate, isobornyl methacrylate, ethyl methacrylate,butyl methacrylate or hydroxyethylmethacrylate, of which methylmethacrylate is particularly preferred.

It is preferred that the ethylenically unsaturated monomers aresubstantially free from vinylidene chloride. If included, the amountthereof is preferably less than 10 wt %, most preferably less than 5 wt%, or even less than 1 wt % of the ethylenically unsaturated monomers.It is also preferred that the ethylenically unsaturated monomers aresubstantially free from any halogen containing monomers. If included,the amount thereof is preferably less than 10 wt %, most preferably lessthan 5 wt %, or even less than 1 wt % of the ethylenically unsaturatedmonomers.

Preferably the ethylenically unsaturated monomers comprise small amountsof one or more crosslinking multifunctional monomers, such as one ormore of divinyl benzene, ethylene glycol di(meth)acrylate, di(ethyleneglycol) di(meth)acrylate, triethylene glycol di(meth)acrylate, propyleneglycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanedioldi(meth)acrylate, glycerol di(meth)acrylate, 1,3-butanedioldi(meth)acrylate, neopentyl glycol di(meth)acrylate, 1,10-decanedioldi(meth)acrylate, pentaerythritol tri(meth)acrylate, pentaerythritoltetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate,triallylformal tri(meth)acrylate, allyl methacrylate, trimethylolpropane tri(meth)acrylate, tributanediol di(meth)acrylate, PEG #200di(meth)acrylate, PEG #400 di(meth)acrylate, PEG #600 di(meth)acrylate,3-acryloyloxyglycol monoacrylate, triacryl formal, triallyl isocyanate,triallyl isocyanurate etc. Particularly preferred are crosslinkingmonomers that are at least tri-functional, examples of which includepentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate,dipentaerythritol hexa(meth)acrylate, triallylformal tri(meth)acrylate,trimethylol propane tri(meth)acrylate, triacryl formal, triallylisocyanate and triallyl isocyanurate. The amounts of crosslinkingfunctional monomers may, for example, be from 0.1 to 10 wt % or from 0.1to 1 wt % or from 1 to 3 wt % of the ethylenically unsaturated monomers,0.1 to 1 wt % being particularly preferred in case the one or moremultifunctional monomers are at least tri-functional and 1 to 3 wt %being particularly preferred in case the one or more multifunctionalmonomers are di-functional.

If ethylenically unsaturated monomers other than acrylonitrile,methacrylonitrile, monomers selected from the group consisting of estersof acrylic acid, esters of methacrylic acid and mixtures thereof, andone or more crosslinking multifunctional monomers, are included, theamount thereof is preferably from 0 to 10 wt %, most preferably from 0to 5 wt %. Examples of such other kinds of monomers that may be includedare nitrile containing monomers such as α-ethoxyacrylonitrile,fumaronitrile or crotonitrile; vinyl pyridine; vinyl esters such asvinyl acetate; styrenes such as styrene, halogenated styrenes orα-methyl styrene; dienes such as butadiene, isoprene and chloroprene;unsaturated carboxylic compounds like acrylic acid, methacrylic acid andsalts thereof; or other unsaturated monomers like acrylamide,methacrylamide or N-substituted maleimides.

In an embodiment of the invention the ethylenically unsaturated monomerssubstantially consist of acrylonitrile, methacrylonitrile, monomersselected from the group consisting of esters of acrylic acid, esters ofmethacrylic acid and mixtures thereof, and one or more crosslinkingmultifunctional monomers. In a particular embodiment thereof theethylenically unsaturated monomers substantially consist ofacrylonitrile, methacrylonitrile, monomers selected from the groupconsisting of esters of acrylic acid, preferably one or more of methylacrylate or ethyl acrylate, and one or more crosslinking multifunctionalmonomers.

The softening temperature of the polymer shell, normally correspondingto its glass transition temperature (T_(g)), is preferably within therange from 0 to 100° C., most preferably from 30 to 90° C.

The propellant is a hydrocarbon or mixture of hydrocarbons preferablyhaving a boiling temperature not higher than the softening temperatureof the thermoplastic polymer shell. The boiling point at atmosphericpressure is preferably within the range from −50 to 100° C., mostpreferably from −20 to 50° C., particularly most preferably from −20 to30° C. The propellant may consist substantially of at least one ofmethane, ethane, propane, isobutane, n-butane and neo-pentane, but mayalso additionally comprise one or more other hydrocarbons, for examplein an amount from 0 to 50 wt % of the propellant. Examples of suchhydrocarbons include, n-pentane, isopentane, cyclopentane, hexane,isohexane, neo-hexane, cyclohexane, heptane, isoheptane, octane andisooctane. Aside from them, other hydrocarbon types can also be used,such as petroleum ether, or chlorinated or fluorinated hydrocarbons,such as methyl chloride, methylene chloride, dichloroethane,dichloroethylene, trichloroethane, trichloroethylene,trichlorofluoromethane, perfluorinated hydrocarbons, fluorine containingethers, etc. Preferred propellants comprise isobutane, alone or in amixture with one or more other hydrocarbons. The amount of isobutane inthe propellant is preferably from 50 to 100 wt %, most preferably from75 to 100 wt %.

T_(start) for the expandable microspheres is preferably from 50 to 110°C., most preferably from 70 to 100° C. T_(max) for the expandablemicrospheres is preferably from 100 to 200° C., most preferably from 120to 170° C.

Apart from the polymer shell and the propellant the microspheres maycomprise further substances added during the production thereof,normally in an amount from 0 to 20 wt %, preferably from 1 to 10 wt %.Examples of such substances are solid suspending agents, such as one ormore of starch, crosslinked polymers, gum agar, derivated cellulose likefor example methyl cellulose, hydroxypropyl methylcellulose,hydroxyethylcellolose and carboxy methylcellulose, silica, colloidalclays like for example chalk and bentonite, and/or one or more salts,oxides or hydroxides of metals like Al, Ca, Mg, Ba, Fe, Zn, Ni and Mn,for example one or more of calcium phosphate, calcium carbonate,magnesium hydroxide, barium sulphate, calcium oxalate, and hydroxides ofaluminium, iron, zinc, nickel or manganese. If present, these solidsuspending agents are normally mainly located to the outer surface ofthe polymer shell. However, even if a suspending agent has been addedduring the production of the microspheres, this may have been washed offat a later stage and could thus be substantially absent from the finalproduct.

The expandable microspheres preferably have a volume median diameterfrom 1 to 500 μm, more preferably from 5 to 50 μm, most preferably from10 to 50 μm. The amount of propellant in the expandable microspheres ispreferably from 5 to 40 wt %, more preferably from 10 to 40 wt %, mostpreferably from 15 to 40 wt %, particularly most preferably from 20 to35 wt %.

The term expandable microspheres as used herein refers to expandablemicrospheres that have not previously been expanded, i.e. unexpandedexpandable microspheres.

A further aspect of the invention concerns a process for the productionof expandable thermoplastic microspheres as described above. The processcomprises polymerising ethylenically unsaturated monomers as describedabove in a preferably aqueous suspension in the presence of a propellantas described above to yield microspheres comprising a polymer shellencapsulating said propellant. Regarding the kinds and amounts ofmonomers and propellant, the above description of the expandablemicrospheres is referred to. The production may follow the sameprinciples as described in the earlier mentioned U.S. Pat. Nos.3,615,972, 3,945,956, 4,287,308, 5,536,756, 6,235,800, 6,235,394 and6,509,384, EP 486080, EP 1054034, EP 1288272 and EP1408097, WO2004/072160 and JP Laid Open No. 1987-286534.

In an embodiment of the invention the microspheres are produced in abatchwise process and the polymerisation may then be conducted asdescribed below in a reaction vessel. For 100 parts of monomer phase(suitably including monomers and propellant, the proportions of whichdetermine proportions of monomers in the polymer shell and the amount ofpropellant in the final product), one or more polymerisation initiator,preferably in an amount from 0.1 to 5 parts, aqueous phase, preferablyin an amount from 100 to 800 parts, and one or more preferably solidcolloidal suspending agent, preferably in an amount from 1 to 20 parts,are mixed and homogenised. The size of the droplets of the monomer phaseobtained determines the size of the final expandable microspheres inaccordance with the principles described in e.g. U.S. Pat. No.3,615,972, that can be applied for all similar production methods withvarious suspending agents. The temperature is suitably maintained from40 to 90° C., preferably from 50 to 80° C., while the suitable pHdepends on the suspending agent used. For example, a high pH, preferablyfrom 5 to 12, most preferably from 6 to 10, is suitable if thesuspending agent is selected from salts, oxides or hydroxides of metalslike Ca, Mg, Ba, Zn, Ni and Mn, for example one or more of calciumphosphate, calcium carbonate, magnesium hydroxide, magnesium oxide,barium sulphate, calcium oxalate, and hydroxides of zinc, nickel ormanganese. A low pH, preferably from 1 to 6, most preferably from 3 to5, is suitable if the suspending agent is selected from starch, methylcellulose, hydroxypropyl methylcellulose, hydroxypropyl methylcellulose,carboxy methylcellulose, gum agar, silica, colloidal clays, or oxide orhydroxide of aluminium or iron. Each one of the above agents hasdifferent optimal pH, depending on, for example, solubility data.

In order to enhance the effect of the suspending agent, it is alsopossible to add small amounts of one or more promoters, for example from0.001 to 1 wt %. Usually, such promoters are organic materials and may,for example, be selected from one or more of water-soluble sulfonatedpolystyrenes, alginates, carboxymethylcellulose, tetramethyl ammoniumhydroxide or chloride or water-soluble complex resinous aminecondensation products such as the water-soluble condensation products ofdiethanolamine and adipic acid, the water-soluble condensation productsof ethylene oxide, urea and formaldehyde, polyethylenimine,polyvinylalcohol, polyvinylpyrrolidone, polyvinylamine, amphotericmaterials such as proteinaceous, materials like gelatin, glue, casein,albumin, glutin and the like, non-ionic materials like methoxycellulose,ionic materials normally classed as emulsifiers, such as soaps, alkylsulfates and sulfonates and long chain quaternary ammonium compounds.

Conventional radical polymerisation may be used and initiators aresuitably selected from one or more of organic peroxides such as dialkylperoxides, diacyl peroxides, peroxy esters, peroxy dicarbonates, or azocompounds. Suitable initiators include dicetyl peroxydicarbonate,di(4-tert-butylcyclohexyl) peroxydicarbonate, dioctanoyl peroxide,dibenzoyl peroxide, dilauroyl peroxide, didecanoyl peroxide, tert-butylperacetate, tert-butyl perlaurate, tert-butyl perbenzoate, tert-butylhydroperoxide, cumene hydroperoxide, cumene ethylperoxide,diisopropylhydroxy dicarboxylate, 2,2′-azobis(2,4-dimethylvaleronitrile), 2,2′-azobis(isobutyronitrile),1,1′-azobis(cyclohexane-1-carbonitrile), dimethyl2,2′-azobis(2-methylpropionate),2,2′-azobis[2-methyl-N-(2-hydroxyethyl)propionamide] and the like. It isalso possible to initiate the polymerisation with radiation, such ashigh energy ionising radiation.

When the polymerisation is essentially complete, microspheres arenormally obtained as an aqueous slurry or dispersion, which can be usedas such or dewatered by any conventional means, such as bed filtering,filter pressing, leaf filtering, rotary filtering, belt filtering orcentrifuging to obtain a so called wet cake. However, it is alsopossible to dry the microspheres by any conventional means, such asspray drying, shelf drying, tunnel drying, rotary drying, drum drying,pneumatic drying, turbo shelf drying, disc drying or fluidisedbed-drying.

If appropriate, the microspheres may at any stage be treated to reducethe amount of residual unreacted monomers, for example by any of theprocedures described in the earlier mentioned WO 2004/072160 or U.S.Pat. No. 4,287,308.

A further aspect of the invention concerns expanded microspheresobtained by expanding expandable microspheres as described above, forexample to a particle diameter from 2 to 5 times larger than thediameter of the unexpanded microspheres. The density of the expandedmicrospheres may, for example, be from 0.005 to 0.06 g/cm³. Theexpansion is effected by heating the expandable microspheres to atemperature above T_(start). The upper temperature limit is set by whenthe microspheres start collapsing and depends on the exact compositionof the polymer shell and the propellant. In most cases a temperaturefrom 80° C. to 150° C. is suitable. The density of the expandedmicrospheres can be controlled by selecting temperature and time for theheating. The expansion can be effected by any suitable means for heatingin any suitable device, as described in e.g. EP 0348372, WO 004/056549or WO 2006/009643.

The expandable and expanded microspheres of the invention are useful invarious application such as paper making, printing inks (such aswaterborne inks, solvent borne inks, plastisols, UV-curing inks etc.e.g. for textile, wall paper etc.), putties, sealants, toy-clays,underbody coatings, adhesives, debonding of adhesives, artificialleather, genuine leather, paint, non-woven materials, paper and board,coatings (e.g anti-slip coating etc.) for various materials such aspaper, board, plastics, metals and textile, explosives, cableinsulations, thermoplastics (such as polyethylene, polyvinyl chloride,and ethylene-vinylacetate) or thermoplastic elastomers (such asstyrene-ethylene-butylene-styrene co-polymer, styrene-butadiene-styreneco-polymer, thermoplastic polyurethanes and thermoplastic polyolefins),styrene-butadiene rubber, natural rubber, vulcanized rubber, siliconerubbers, thermosetting polymers (such as epoxies, polyurethanes andpolyesters). In some of these applications expanded microspheres areparticularly advantageous, such as in putties, sealants, toy-clays,genuine leather, paint, explosives, cable insulations and thermosettingpolymers (like epoxies, polyurethanes and polyesters). In some cases itis also possible to use a mixture of expanded and expandablemicrospheres of the invention, for example in underbody coatings,silicone rubbers and light weight foams.

Still a further aspect of the invention concerns an aqueous slurrycomprising expandable thermoplastic microspheres as described above,preferably in an amount from 5 to 55 wt %, most preferably from 20 to 55wt %. Such a slurry is useful for various applications of the expandablemicrospheres, including e.g. paper making. The slurry preferably alsocomprises at least one thickener, preferably compatible with papermaking. Examples of such thickeners include at least partially watersoluble polymers selected from the group consisting of starch, gums,celluloses, chitins, chitosans, glycans, galactans, pectins, mannans,dextrins, co-polymers made from monomers comprising acrylic acid orsalts thereof (preferably up to 50 mol %, most preferably up to 20 mol %acrylic acid or salt thereof), homo- and co-polymers made from monomerscomprising esters or amides of acrylic acid, homo and co-polymers madefrom monomers comprising methacrylic acid, esters or amides thereof,rubber latexes, poly(vinyl chloride) and copolymers, poly(vinyl esters)and co-polymers (e.g. with ethylene), poly(vinyl alcohol), polyamines,polyetyleneimine, polyethylene/polypropylene oxides, polyurethane, andaminoplast and phenoplast precondensates such as urea/formaldehyde,urea/melamine/formaldehyde or phenol/formaldehyde and polyamidoamineepichlorohydrin resins. Examples of suitable gums include guar gums,tamarind gums, locust bean gums, tare gums, karaya, okra, acacia,xanthan gums etc. and mixtures thereof, of which guar gums areparticularly preferred. Examples of suitable celluloses includederivatives such as optionally chemically modified CMC (carboxymethylcellulose) and cellulose ethers like EHEC (ethyl hydroxyethyl cellulose)and HEC (hydroxyethyl cellulose), and mixtures thereof. Chemicallymodified cellulose derivatives include, for example, those modified withvarious functional groups such as quaternary amines, other amines,sulphates, sulphonates, phosphates, phosphonates, polyethylene oxide andpolypropylene oxide.

The at least partially water soluble polymer can be straight chained,branched or cross-linked. The average molecular weight can vary withinwide limits, depending on the kind of polymer. In most cases thepreferred average molecular weight is at least 500, more preferably atleast 2000 and most preferably at least 5000. The upper limit is notcritical and in most cases the average molecular weight is preferably upto 50 000 000, more preferably up to 10 000 000, most preferably up to 1000 000.

Particularly preferred polymers include starch, CMC, EHEC, Guar gum,polyamidoamine epichlorohydrin resins, co-polymers of acrylic acid withother monomers (e.g. with acrylamide), and homo- or co-polymers ofpolyacrylamides, polyamine, poly(vinyl alcohol) andpolyethylene/polypropylene oxides.

One or more at least partially water soluble polymers effective asthickener are preferably present in an amount to stabilise the slurryagainst substantial sedimentation or flotation of the microspheres to anextent that they cannot be re-dispersed again. In many cases this can beachieved by adding sufficient polymer to obtain a preferred viscosity ofthe slurry from about 150 to about 1000 mPas at 25° C., most preferablyfrom about 200 to about 600 mPas at 25° C. (refers to measurement withan Anton Paar DV-1P viscosimeter equipped with a spindle L3). The amountrequired to stabilise the slurry depends on the polymer and othercircumstances such as the pH. In many cases a preferred content of atleast partially water soluble polymer in the slurry is from about 0.1 toabout 15 wt %, most preferably from about 0.1 to about 10 wt %,particularly most preferably from about 0.5 to about 10 wt %.

All thickeners and other additives disclosed in any of the earliermentioned WO 2006/068573 and WO 2006/068574 may be used in the aqueousslurry of the invention in the preferred amounts also disclosed therein.

Particular aspects of the invention relates to the use of expandablemicrospheres as described above in printing inks and in the productionof paper from a stock containing cellulosic fibres, artificial leatherand nonwoven.

When used in printing inks, in particular water borne printing inks, theexpandable microspheres, preferably wet unexpanded microspheres, areadded to standard formulations well known to those skilled in the art.Such formulations usually include one or more binders and one or morethickeners. Other components may include, for example, pigments,anti-foaming agents, fillers, chemicals for preventing skinning orclogging, etc. Printing inks may also be based on acrylate dispersionsor plastisols comprising expandable microspheres. After printing, themicrospheres are expanded by heating, before, during or after the dryingof the ink. Such printing inks are particularly suitable for printing ontextiles or wallpaper.

When used in artificial leather, the expandable microspheres, preferablydry unexpanded microspheres, are used in standard formulations instandard procedures known by those skilled in the art, for example inthe surface layer of multilayer artificial leather, for example of suedetype or any other kind of structure. The artificial leather can beproduced by any standard process, such as Release paper process, Directcoating of woven or non-woven, or the Coagulation process, from anystandard material such polyurethane (PU), polyvinyl chloride (PVC) andmixtures thereof. Usually, artificial leather produced by any of theabove processes is coated with PU or PVC paste containing expandablemicrospheres and is then heated to expand the microspheres.

In production of paper the expandable microspheres are preferably usedto increase the bulk of the paper, but may alternatively serve otherpurposes. The microspheres are then preferably added to a stockcontaining cellulosic fibres, which then is dewatered and dried, whereinthe microspheres expand. In most cases the expansion contributes toincreasing the bulk of the paper.

A special aspect of the invention relates to a process for theproduction of paper comprising the steps of adding thermally expandablemicrospheres as described above to a stock containing cellulosic fibres,dewatering the stock on a wire to obtain paper, and drying the paper byapplying heat and thereby also raising the temperature of themicrospheres sufficiently for them to expand and increase the bulk ofthe paper.

The amount of expandable microspheres added to the stock is preferablyfrom 0.1 to 20 wt %, most preferably from 0.2 to 10 wt % drymicrospheres of the dry content in the stock. Any kind of paper machineknown in the art can be used.

The term “paper”, as used herein, is meant to include all types ofcellulose-based products in sheet or web form, including, for example,board, cardboard and paper board. The invention has been foundparticularly advantageous for the production of board, cardboard andpaper board, particularly with a basis weight from 50 to 1000 g/m²,preferably from 150 to 800 g/m².

The paper may be produced as a single layer or a multi-layer paper. Ifthe paper comprises three or more layers, the expandable microspheresmay be added to the portions of the stock forming one or several ofthese layers, for example only to the portions of the stock not formingany of the two outer layers.

The stock preferably contains from 50 to 100 wt %, most preferably from70 to 100 wt % of cellulosic fibres, based on dry material. Beforedewatering, the stock besides expandable microspheres, may also containone or more fillers, e.g. mineral fillers like kaolin, china clay,titanium dioxide, gypsum, talc, chalk, ground marble or precipitatedcalcium carbonate, and optionally other commonly used additives, such asretention aids, sizing agents, aluminium compounds, dyes, wet-strengthresins, optical brightening agents, etc. Examples of aluminium compoundsinclude alum, aluminates and polyaluminium compounds, e.g. polyaluminiumchlorides and sulphates. Examples of retention aids include cationicpolymers, anionic inorganic materials in combination with organicpolymers, e.g. bentonite in combination with cationic polymers orsilica-based sols in combination with cationic polymers or cationic andanionic polymers. Examples of sizing agents include cellulose reactivesizes such as alkyl ketene dimers and alkenyl succinic anhydride, andcellulose non-reactive sizes such as rosin, starch and other polymericsizes like copolymers of styrene with vinyl monomers such as maleicanhydride, acrylic acid and its alkyl esters, acrylamide, etc.

At drying, the paper, and thereby also the microspheres, is preferablyheated to a temperature from 50 to 150° C., most preferably from 60 to110° C. This results in expansion of the microspheres and thereby also abulk increase of the paper. The magnitude of this bulk increase dependson various factors, such as the origin of cellulosic fibres and othercomponents in the stock, but is in most cases from 5 to 70% or more perweight percentage of retained microspheres in the dried paper, comparedto the same kind of paper produced without addition of expandablemicrospheres or any other expansion agent. Any conventional means ofdrying involving transferring heat to the paper can be applied, such ascontact drying (e.g. by heated cylinders), forced convection drying(e.g. by hot air), infrared techniques, or combinations thereof. In thecase of contact drying, the temperature of the contact surfaces, e.g.the cylinders, is preferably from 20 to 150° C., most preferably from 30to 130° C. The paper may pass a series of several cylinders, e.g. up to20 or more, of increasing temperature.

The cellulosic fibres in the stock may, for example, come from pulp madefrom any kind of plants, preferably wood, such as hardwood and softwood.The cellulosic fibres may also partly or fully originate from recycledpaper, in which case the invention has been found to give unexpectedlygood results.

The expandable microspheres can be added in any form, although it from apractical point of view is most preferred to add them in the form of anaqueous slurry as described above.

The invention will be further described in connection with the followingExamples which, however, are not to be interpreted to limit the scope ofthe invention. If not otherwise stated, all parts and percentages referto parts and percent by weight.

The expansion properties of the microspheres were evaluated on a MettlerTMA 40 with a TC15 TA processor and a PC with STAR^(e) software using aheating rate of 20° C./min and a load (net.) of 0.06 N. T_(start) is thetemperature at which the expansion starts, T_(max) is the temperature atwhich maximum expansion is obtained and TMA-density is the density ofthe microspheres at T_(max).

The particle size and size distribution was determined by laser lightscattering on a Malvern Mastersizer Hydro 2000 SM apparatus on wetsamples. The mean particle size is presented as the volume mediandiameter d(0.5).

The amount of propellant was determined by thermal gravimetric analysis(TGA) on a Mettler Toledo TGA/SDTA851e. All samples were dried prior toanalysis in order to exclude as much moisture as possible and if presentalso residual monomers. The analyses were performed under an atmosphereof nitrogen using a heating rate at 20° C. min⁻¹ starting at 30° C.

EXAMPLE 1

A reaction mixture containing Mg(OH)₂-stabilised organic droplets inwater was created by mixing the phases and stirring vigorously until asuitable droplet size had been achieved. The water dispersion contained3.2 parts of Mg(OH)₂ and 331 parts of water. The organic dropletscontained 2.0 parts of dilaurylperoxide, 38 parts of isobutane, 52.0parts of acrylonitrile, 28.0 parts of methacrylonitrile, 20.0 parts ofmethyl acrylate and 0.3 parts of trimethylolpropane trimethacrylate.Polymerisation was performed at 62° C. in a sealed reactor underagitation. After cooling to room temperature a sample of the obtainedmicrosphere slurry was removed for determination of the particle sizedistribution. After filtration, washing and drying the particles wereanalysed by TMA. The dry particles contained about 23% by weight ofisobutane and had a mean particle size of about 34 μm. The TMA-resultsare found in Table 1.

EXAMPLES 2-24

Microspheres were prepared in a plurality of polymerisation experimentsperformed as in Example 1 except for monomers and propellants, whichwere added according to Table 1. The amounts of water and Mg(OH)₂ in theexamples varied between 235-365 parts and 2.2-4.8 parts respectively.This is due to small differences in the recipes in differentpolymerisation reactors but does not influence the thermal properties ofthe polymerised particles. In Examples 3, 4, 7 and 10, prior to handlingthe particles outside the reactor, the amount of residual monomers wasreduced by treatment with 6 parts NaHSO₃ for 5 hrs at 70° C., afterwhich the temperature was lowered to room temperature and the particleswere isolated and analysed. In Example 2 the amount of residual monomerswas reduced by treatment with 3.5 parts NaHSO₃ and in Examples 18 and 22with 1.4 parts NaHSO₃. In Example 17 the amount of residual monomers wasreduced by treatment with 8.6 parts of 25% NH₃ and 30 parts of water for1 hr at 70° C. followed by addition of 1.5 parts of (NH₄)₂S₂O₈ dissolvedin 17 parts of water and continued reaction for 4 h at 70° C. afterwhich the temperature was lowered to room temperature and the particleswere isolated and analysed. Analytical results can be found in Table 1.

TABLE 1 Analytical results for Examples 1-24 and amounts of differentchemicals used, expressed as parts per weight. TMA- Size PropellantT_(max) density Example AN MAN X IB IP (μm) (wt %) T_(start) (° C.) (°C.) (g/l)  1, X = MA 52 28 20 38 — 34 23 90 163 4.0  2, X = MA 60 30 1034 — 34 27 104 177 6.7  3, X = MA 45 25 30 38 — 40 22 93 151 5.2  4, X =MA 60 20 20 38 — 36 21 97 156 8.9  5, X = EA 62 33 5 35 — 29 17 106 1858.5  6, X = EA 59 31 10 35 — 25 22 98 176 5.1  7, X = EA 52 28 20 38 —30 16 94 162 6.5  8, X = BA 62 33 5 35 — 28 19 104 184 13.2  9, X = BA59 31 10 35 — 28 18 97 171 11.5 10, X = BA 52 28 20 38 — 28 22 95 14923.0 11, X = MMA 52 28 20 38 — 25 20 92 157 5.3 12, X = EMA 52 28 20 38— 27 11 90 150 9.7 13, X = BMA 52 28 20 38 — 25 12 94 137 45.4 14 65 35— — 35 32 25 115 185 5.2 15, X = MA 52 28 20 — 23 34 19 121 156 7.2 16,X = MMA 52 28 20 — 35 27 26 112 151 7.9 17 65 35 — 34 — 44 25 113 1868.4 18, X = MA 52 28 20 33 — 12 21 100 141 9.1 19, X = MMA 52 28 20 34 —15 18 99 140 14.6 20, X = MA 52 28 20 35 — 28 23 92 158 4.5 21, X = MA65 15 20 35 — 30 19 93 160 9.9 22, X = MA 45 25 30 33 — 12 20 95 135 9.023, X = MA 52 28 20 38 — 23 20 94 159 5.1 24, X = MA 55 30 15 38 39 2698 180 4.3 AN = acrylonitrile, MAN = methacrylonitrile, MA = methylacrylate, EA = ethyl acrylate, BA = butyl acrylate, MMA = methylmethacrylate, EMA = ethyl methacrylate, BMA = butyl methacrylate, IB =isobutane, IP = isopentane

The brightness of the dry microspheres from Examples 18 and 19 wasanalysed according to ISO 2470 with a Zeiss Elrepho Reflectometer;measurement of diffuse blue reflectance factor, light with a wavelengthof 457 nm and using reference paper 59.65. However, due to the need of asample holder for powders, the reflectance of the microspheres couldonly be measured through a glass disc, giving a decrease of thereflectance of about 11% (units of percent). Thus, the numbers are givenwith the reduction of the reflectance included, meaning that the truevalues for the brightness are about 11 units of percent higher. AsReference a commercial microsphere product was used having a polymershell of 58% vinylidene chloride, 33% acrylonitrile and 9% methylmethacrylate and isobutane as propellant. The results can be found inTable 2.

TABLE 2 Brightness of microspheres Brightness (%) Example 18 75.9Example 19 78.6 Reference 60.1

Dry microspheres from Examples 20 and 21 were tested in respect ofsolvent resistance. Each sample was mixed with a solvent mixtureconsisting of 2-butanone and DMF (90/10 w/w) and maintained for 7 daysat room temperature. After this treatment the microspheres werefiltrated and dried and analysed with TMA again to see how the expansionbehaviour had been affected. As a Reference, a commercial microsphereproduct having a polymer shell of 22% vinylidene chloride, 60%acrylonitrile and 18% methyl methacrylate and isobutane as propellantwas treated the same way. The results are shown in Table 3.

TABLE 3 Resistance against a mixture of 2-butanone/DMF 90/10 Expansionafter Expansion before exposure to solvent exposure to solvent during 1week TMA- TMA- Micro- T_(start) T_(max) density T_(start) T_(max)density spheres (° C.) (° C.) (g/l) (° C.) (° C.) (g/l) Comment Example20 92 158 4.5 73 161 44.9 Example 21 93 160 9.9 97 161 13.6 Reference 99150 12.3 107 148 106.2 CollapsesIt appears that the microspheres of the invention are considerably lessaffected by the solvent mixture than the Reference microspheres madefrom high amounts of vinylidene chloride monomers. It can also be seenthat the microspheres of Example 21 made from higher content ofacrylonitrile monomers have the best solvent resistance.

EXAMPLE 25

A single layer paper board with a basis weight of about 300 g/m² wasproduced in a pilot paper machine with a machine speed of 4 m/min andnot having recirculated process water. The pulp was composed of 42.5 wt% hardwood, 42.5 wt % softwood pulp and 15.0 wt % filler of GCC (groundcalcium carbonate) and was beaten to a Schopper-Riegler value of 25° SRand then dispersed to give a pulp slurry/stock. An aqueous slurry ofexpandable microspheres was before the mixing box added to the stock inan amount of about 2.0 wt % dry microspheres of the dry substance in thestock. As retention aid Compozil® was used and AKD was used as sizingagent. In the drying section the paper web was heated by cylindershaving a temperature profile from 65 to 122° C. Expandable microspheresfrom Examples 3, 4, 18 and 22 were tested. Gohseran L-3266™ (sulfonicacid modified polyvinylalcohol) was added to the microsphere slurries tostabilise against flotation or sedimentation. Commercially availablemicrosphere slurries, with microspheres having a polymer shell of 73%vinylidene chloride, 24% acrylonitrile and 3% methyl methacrylate andhaving isobutane as propellant, and with Solvitose C5™ (starch) fromAvebe Starches North Europe as thickening agent, were tested asReference microspheres. In order to determine the retention of themicrospheres, paper samples were taken before the press section fordetermination of the amount of microspheres (using GC). The retentionwas calculated from the microspheres addition and the content ofmicrospheres in the paper. Moreover, samples from the dried paper weretaken for determination of bulk and thickness. The results are shown inTable 4.

In the same way a single layer paper board with a basis weight of about80 g/m² was produced. Microspheres of Examples 3 and 4 were tested alongwith the Reference microspheres. The results are shown in Table 5.

TABLE 4 Basis weight of about 300 g/m² AN/MAN/MA in Amount of Increasedbulk polymer shell propellant Particle size Retention (% per percentageof (wt %) (wt %) (μm) (%) retained microspheres 45/25/30 (Ex.22) 20 1228 20 52/28/20 (Ex.18) 21 12 21 10 45/25/30 (Ex.3) 22 40 39 71 60/20/20(Ex.4) 21 36 50 54 Ref 1 VDC/AN/MMA 14 14 78 16 Ref 2 VDC/AN/MMA 17 2070 23 AN = acrylonitrile, MAN = methacrylonitrile, MA = methyl acrylate,VDC = vinylidene chloride, MMA = methyl methacrylate

TABLE 5 Basis weight of about 80 g/m² AN/MAN/MA Amount of Increased bulkpolymer shell propellant Particle Retention (% per percentage of (wt %)(wt %) size (μm) (%) retained microspheres) 45/25/30 (Ex.3) 22 40 47 5960/20/20 (Ex.4) 21 36 44 72 Ref 1 VDC/AN/MMA 14 14 77 9 Ref 2 VDC/AN/MMA17 20 57 21 AN = acrylonitrile, MAN = methacrylonitrile, MA = methylacrylate; VDC = vinylidene chloride, MMA = methyl methacrylateThe results show that the overall trend is that the increases of thebulk of the paper from the chlorine-free microspheres of the inventionare comparable with the increases of the bulk from the chlorinecontaining microspheres. It also appears that a large particle diametergives a very high increase of the bulk.

EXAMPLE 2

Microspheres from Examples 11 and 23 were tested in PVC plastisol bymixing 4% by weight of dry microspheres using a dissolver disc with apre-mixed plastisol formulation consisting of 100 parts of PVC (PevikonP 682 from Hydro), 57 parts of diisononyl phthalate, 3 parts ofepoxidized soybean oil (Edenol D81 from Cognis), 1 part of barium-zincstabilizer (Mark BZ 505 from Crompton), and 3 parts of butylbenzylphthalate. Draw downs were made with a film applicator with a 250 μmgap. One of the draw downs was gelated at 100° C. for 45 sec and thethickness of this unexpanded draw down was 160 μm, measured with acoating thickness gauge, Elcometer 355 Standard. The other draw downswere expanded for 60 sec at 140-200° C. and the thicknesses weremeasured in the same way and the expansion factors were calculated bydividing with the thickness of the unexpanded layer. A Mathis labdryerhot air oven was used for both gelation and expansion. A Commerciallyavailable microsphere grade having a polymer shell of 58% acrylonitrileand 42% methyl methacrylate and having isobutane as propellant, wastested in the same way as. The expansion factors are presented in Table6.

TABLE 6 Expansion factors for microspheres in PVC plastisol Temperature(° C.) Example 11 Example 23 Reference 140 2.3 3.7 1.8 150 3.4 4.8 2.3160 4.1 5.3 2.6 170 4.3 5.6 2.5 180 4.3 5.9 2.4 190 4.6 5.9 2.3 200 4.75.6 2.1The results show that the expansion factors of the plastisol from themicrosphere of the invention are higher compared to the expansionfactors from the reference microspheres.

1. Thermally expandable thermoplastic microspheres comprising a polymershell made from ethylenically unsaturated monomers encapsulating apropellant, said ethylenically unsaturated monomers consistingessentially of from 40 to 70 wt % of acrylonitrile, from 5 to 40 wt % ofmethacrylonitrile, from above 20 to 50 wt % of acrylate monomersselected from the group consisting of methyl acrylate, ethyl acrylateand mixtures thereof, and from 0.1 to 10 wt % of one or morecrosslinking multifunctional monomers, and said propellant comprisingfrom 50 to 100 wt % of isobutane.
 2. Microspheres as claimed in claim 1,wherein said ethylenically unsaturated monomers comprises from 45 to 65wt % acrylonitrile.
 3. Microspheres as claimed in claim 1, wherein saidethylenically unsaturated monomers comprises from 10 to 35 wt % ofmethacrylonitrile.
 4. Microspheres as claimed in claim 1, wherein saidpropellant comprises from 75 to 100 wt % isobutane.
 5. Microspheres asclaimed in claim 4, wherein said propellant consists essentially ofisobutane.
 6. Microspheres as claimed in claim 5, wherein saidpropellant consists of isobutane.
 7. Microspheres as claimed in claim 1,wherein said ethylenically unsaturated monomers comprises from above 20to 50 wt % of methyl acrylate.
 8. Microspheres as claimed in claim 1,wherein said ethylenically unsaturated monomers are substantially freefrom or comprise less than 10 wt % of halogen containing monomers. 9.Microspheres as claimed in claim 1, wherein T_(start) is from 50 to 110°C.
 10. Thermally expandable thermoplastic microspheres comprising apolymer shell made from ethylenically unsaturated monomers encapsulatinga propellant, said ethylenically unsaturated monomers consistingessentially of from 45 to 65 wt % of acrylonitrile, from 10 to 35 wt %of methacrylonitrile, from above 20 to 50 wt % of acrylate monomersselected from the group consisting of methyl acrylate, ethyl acrylateand mixtures thereof, and from 0.1 to 10 wt % of one or morecrosslinking multifunctional monomers, and said propellant comprisingfrom 50 to 100 wt % isobutane.
 11. Process for the production ofthermally expandable microspheres comprising polymerising ethylenicallyunsaturated monomers in the presence of a propellant to yieldmicrospheres comprising a polymer shell encapsulating said propellant,said ethylenically unsaturated monomers consisting essentially of from40 to 70 wt % acrylonitrile, from 5 to 40 wt % methacrylonitrile, fromabove 20 to 50 wt % of acrylate monomers selected from the groupconsisting of methyl acrylate, ethyl acrylate and mixtures thereof, andfrom 0.1 to 10 wt % of one or more crosslinking multifunctionalmonomers, and said propellant comprising from 50 to 100 wt % ofisobutane.
 12. Aqueous slurry comprising thermally expandablethermoplastic microspheres comprising a polymer shell made fromethylenically unsaturated monomers encapsulating a propellant, saidethylenically unsaturated monomers consisting essentially of from 40 to70 wt % of acrylonitrile, from 5 to 40 wt % of methacrylonitrile, fromabove 20 to 50 wt % of acrylate monomers selected from the groupconsisting of methyl acrylate, ethyl acrylate and mixtures thereof, andfrom 0.1 to 10 wt % of one or more crosslinking multifunctionalmonomers, and said propellant comprising from 50 to 100 wt % ofisobutane.
 13. Aqueous slurry as claimed in claim 12 further comprisingat least one thickener being an at least partially water soluble polymerselected from the group consisting of starch, gums, celluloses, chitins,chitosans, glycans, galactans, pectins, mannans, dextrins, co-polymersmade from monomers comprising acrylic acid or salts thereof, homo- andco-polymers made from monomers comprising esters or amides of acrylicacid, homo and co-polymers made from monomers comprising methacrylicacid, esters or amides thereof, rubber latexes, poly(vinyl chloride) andcopolymers, poly(vinyl esters) and co-polymers, poly(vinyl alcohol),polyamines, polyetyleneimine, polyethylene/polypropylene oxides,polyurethane, aminoplast, phenoplast precondensates and polyamidoamineepichlorohydrin resins.
 14. Expanded microspheres obtained by expandingthermally expandable thermoplastic microspheres comprising a polymershell made from ethylenically unsaturated monomers encapsulating apropellant, said ethylenically unsaturated monomers consistingessentially of from 40 to 70 wt % of acrylonitrile, from 5 to 40 wt % ofmethacrylonitrile, from above 20 to 50 wt % of acrylate monomersselected from the group consisting of methyl acrylate, ethyl acrylateand mixtures thereof, and from 0.1 to 10 wt % of one or morecrosslinking multifunctional monomers, and said propellant comprisingfrom 50 to 100 wt % of isobutane.
 15. Printing ink comprising thermallyexpandable thermoplastic microspheres comprising a polymer she made fromethylenically unsaturated monomers encapsulating a propellant, saidethylenically unsaturated monomers consisting essentially of from 40 to70 wt % of acrylonitrile, from 5 to 40 wt % of methacrylonitrile, fromabove 20 to 50 wt % of acrylate monomers selected from the groupconsisting of methyl acrylate, ethyl acrylate and mixtures thereof, andfrom 0.1 to 10 wt % of one or more crosslinking multifunctionalmonomers, and said propellant comprising from 50 to 100 wt % ofisobutane.
 16. Process for the production of paper comprising the stepsof adding thermally expandable thermoplastic microspheres to a stockcontaining cellulosic fibres, dewatering the stock on a wire to obtainpaper, and drying the paper by applying heat and thereby also raisingthe temperature of the microspheres sufficiently for them to expand andincrease the bulk of the paper, said expandable thermoplasticmicrospheres comprising a polymer shell made from ethylenicallyunsaturated monomers encapsulating a propellant, said ethylenicallyunsaturated monomers consisting essentially of from 40 to 70 wt % ofacrylonitrile, from 5 to 40 wt % of methacrylonitrile, from above 20 to50 wt % of acrylate monomers selected from the group consisting ofmethyl acrylate, ethyl acrylate and mixtures thereof, and from 0.1 to 10wt % of one or more crosslinking multifunctional monomers, and saidpropellant comprising from 50 to 100 wt % of isobutane.
 17. Process asclaimed in claim 16, wherein the thermally expandable microspheres areadded in the form of an aqueous slurry comprising said thermallyexpandable microspheres.